1. Field of the Invention
The present invention relates to an electric power steering system which assists a steering operation by applying a driving force from an electric motor to a steering mechanism, the electric motor being controlled for driving thereof in accordance with a steering torque.
2. Description of Related Art
An electric power steering system which assists a steering operation by transmitting a driving force from an electric motor to a steering mechanism is conventionally mounted in a motor vehicle. The electric motor is feedback-controlled on the basis of a target electric current determined in accordance with a steering torque applied to a steering wheel, whereby a steering assist force according to the steering torque is applied to the steering mechanism.
When a motor vehicle travels along a mountain road having continuous meanders, for example, the motor vehicle experiences a so-called slalom traveling state. During the slalom traveling, the steering angle and the steering torque tend to be greater than during ordinary traveling. This increases a burden on a driver even with the steering assist, so that an improvement is required for providing a more satisfactory steering feeling.
It is an object of the present invention to provide an electric power steering system which alleviates the burden on the driver during the slalom traveling for the improvement of the steering feeling.
The present invention is directed to an electric power steering system which is adapted to control an electric motor on the basis of a target electric current determined in accordance with a steering torque applied to an operation member and apply a driving force from the electric motor to a steering mechanism for steering assist. The system comprises: a steering torque sensor for detecting the steering torque; a target current setting circuit for determining the target electric current in accordance with the steering torque detected by the steering torque sensor on the basis of an assist characteristic indicative of a relationship between the steering torque and the target electric current; a motor controlling circuit for controlling the electric motor for driving thereof on the basis of the target electric current determined by the target current setting circuit; a traveling state judging circuit for judging whether or not a motor vehicle equipped with the electric power steering system is in a slalom traveling state; and an assist characteristic modifying circuit for offsetting the assist characteristic from an initial characteristic level when the traveling state judging circuit judges that the motor vehicle is in the slalom traveling state.
In accordance with the present invention, the assist characteristic which is employed as a basis for the determination of the target electric current according to the steering torque is modified to be offset from the initial characteristic level when the motor vehicle is in the slalom traveling state. During the slalom traveling, a steering assist force is generated, which is different from that to be generated when the motor vehicle is in any other traveling state.
That is, when the motor vehicle is in the slalom traveling state, the assist characteristic is offset from the initial characteristic level to cause the electric motor to generate a greater driving force than with the initial characteristic level, whereby a greater steering assist force is applied to the steering mechanism than in an ordinary traveling state. Thus, a slalom steering operation can advantageously be assisted, in which a steering direction is frequently changed and a steering angle is changed by greater degrees. Therefore, the burden on the driver can be alleviated for the improvement of the steering feeling.
The traveling state judging circuit maybe adapted to judge that the motor vehicle is in the slalom traveling state if a frequency at which the steering torque detected by the steering torque sensor increases to not smaller than a first predetermined torque level (e.g., 3 Nm) reaches a predetermined reference frequency level. Thus, the slalom traveling state can assuredly be detected when a greater steering torque level is detected at a higher frequency.
The traveling state judging circuit may include a circuit which is adapted to judge that the motor vehicle is brought out of the slalom traveling state when the steering torque detected by the steering torque sensor is continuously kept at not greater than a second predetermined torque level (which is preferably smaller than the first predetermined torque level, e.g., 2 Nm) for not shorter than a first time period (e.g., 30 seconds).
Thus, the fact that the motor vehicle is brought out of the slalom traveling state can be detected on the basis of the detected steering torque level.
The assist characteristic modifying circuit preferably includes a forward stroke steering judging circuit for judging whether or not a forward stroke steering operation is performed, and is adapted to offset the assist characteristic from the initial characteristic level in response to the judgment that the forward stroke steering operation is performed.
With this arrangement, a greater steering assist force can be applied to the steering mechanism when the forward stroke steering operation is performed during the slalom traveling. Therefore, the burden on the driver can effectively be alleviated.
The forward stroke steering judging circuit preferably includes a circuit which is adapted to judge that the forward stroke steering operation is performed when the steering torque detected by the steering torque sensor is continuously kept at not smaller than a third predetermined torque level (which is preferably smaller than the first predetermined torque level and may be substantially equal to the second predetermined torque level, e.g., 2 Nm) for not shorter than a second time period (which is preferably shorter than the first time period, e.g., 0.1 second).
With this arrangement, the forward stroke steering operation of the operation member during the slalom traveling is detected on the basis of the detected steering torque level, and the assist characteristic is modified so as to generate a greater steering assist force in this state.
The assist characteristic modifying circuit is preferably adapted to sequentially modify the assist characteristic in accordance with a change in the steering torque detected by the steering torque sensor.
With this arrangement, the assist characteristic is sequentially modified in accordance with the change in the steering torque during the slalom traveling. Therefore, the steering assist force can properly be generated at any time during the slalom traveling.
The assist characteristic modifying circuit preferably includes a return stroke steering judging circuit for judging whether or not a return stroke steering operation is performed, and is adapted to return the assist characteristic to the initial characteristic level when it is judged that the return stroke steering operation is performed.
With this arrangement, the steering assist force is reduced to increase the steering torque during the return stroke steering operation, so that a proper return steering feeling can be provided for the improvement of the steering feeling.
The return stroke steering judging circuit preferably includes a circuit which is adapted to judge that the return stroke steering operation is performed when a time-based differential value of the steering torque detected by the steering torque sensor decreases to smaller than a predetermined value (e.g., xe2x88x9210 Nm/second).
With this arrangement, a state where the operation member is relatively quickly returned to a neutral position for the return stroke steering operation during the slalom traveling is detected on the basis of the time-based differential value of the steering torque, and the assist characteristic is returned to the initial characteristic level in this state.
The return stroke steering judging circuit may include a circuit which is adapted to judge that the return stroke steering operation is performed when the steering torque detected by the steering torque sensor is continuously kept reduced for longer than a third time period (which is preferably shorter than the first time period and substantially equal to the second time period, e.g., 0.1 second).
With this arrangement, a state where the operation member is relatively slowly returned to the neutral position for the return stroke steering operation during the slalom traveling is detected on the basis of the detected steering torque value, and the assist characteristic is returned to the initial characteristic level in this state.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.